Update on screen brightness handling for Wayland

[Martin Graesslin]

Hi all,

at Akademy we discussed some plans on how to handle screen brightness on 
Wayland. Unfortunately I must say that this doesn't work at all as we imagined 
:-(

I had a look at what Weston is doing for brightness. It uses a forked 
"libbacklight" which internally interacts with the sysfs brighness helpers.

I integrated this into KWin and tried to change backlight. But this obviously 
did not work as only root user is allowed to write to the sysfs file. I then 
tried to open it with the logind wrapper, but logind doesn't have support for 
this kind of files, so that unfortunately doesn't work either.

And here, poor fool, I stand once more, no wiser than I was before. We have a 
file only root can write to, but we are not root. Adding a KAuth helper to 
KWin would not gain us anything compared to having it in powerdevil.

So I suggest that we drop the idea that KWin is responsible for backlight and 
move this feature completely to powerdevil and use the KAuth helper.

Cheers
Martin
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